A deflection yoke deflects an electron beam emitted from an electron gun of a cathode ray tube (CRT) in order to display a screen, and includes a pair of horizontal deflecting coils and a pair of vertical deflecting coils.
FIG. 1 shows a configuration of a deflection yoke. Sub-deflection yoke 1 is provided at a rear end of main deflection yoke 2 having the horizontal deflecting coils and the vertical deflecting coils towards the electron gun. A pair of centering magnets 3 are provided on cover 4 which covers the sub-deflecting coil. The deflection yoke is fastened to CRT 6 with metallic fastening band 5 attached to the cover 4. CRT 6 has straight portion 6a towards the electron gun and substantially-conical portion 6b towards a screen surface.
FIG. 2 shows an operation of the deflection yoke. The deflection yoke is fitted on CRT 6, and forms horizontally and vertically deflecting fields that create a picture on the screen surface of CRT. In a projector, such as a projection type receiver and a projector, plural number of lens 7 (a single lens in FIG. 2) provided at the screen surface of CRT 6 expands a picture on the screen surface and projects picture 9 on screen 8.
FIG. 3 shows a distortion of the picture to be corrected by the sub-deflection yoke 1. The projector includes CRTs of three colors, red, green and blue, which provides a color picture by having three pictures of three colors overlap on one another. In order to having the three pictures from different angles overlap without a displacement, the sub-deflecting coil 1 corrects the position of each picture and pincushion distortion shown in FIG. 3, thereby aligning the three color pictures.
A conventional sub-deflection yoke disclosed in Utility Model Laid-Open Publication No.63-95160 and Japanese Patent Laid-Open Publication No. 3-257742 shown in FIG. 4, and an another conventional sub-deflection yoke shown in FIG. 5 include ring-shaped ferrite core 10, horizontal sub-deflecting coil 11, and vertical sub-deflecting coil 12 which are wound toroidally on core 10. Glass tube 13 of CRT passes through the center of the core 10.
FIG. 6 shows centering magnet 3 for aligning respective centers of pictures of three colors, red, green and blue on the screen 8. The paired centering magnets 3 magnetized to have two poles, N pole and S pole, are provided at the cover 4 with fitting rib 4a provide at the cover 4 of the deflection yoke.
FIG. 7 shows one of conventional centering magnets 3. The centering magnet 3 includes knob portion 3b and knob portion 3c on ring body 3a. The centering magnet 3 is magnetized to have N pole near knob portion 3b and S pole near knob portion 3c, and produces magnetic lines 14 inside the ring body 3a. 
FIG. 8 shows the conventional centering magnets 3. The centering magnet 3 can change the strength of the magnetic lines by superposing magnetic lines 14-1 generated inside ring body 3a-1 on magnetic lines 14-2 generated inside ring body 3a-2, and change the direction of the magnetic line by rotating the centering magnets.
The centering magnet disclosed in Japanese Patent Laid-open Publication No. 2002-75250 is manufactured by injection-molding of plastic resin mixed with ferromagnetic material powder, and then polarizing it. The ferromagnetic material powder may include Alnico-based metal, which has a coercive force of magnetic lines varying little according to a temperature.
FIG. 9 shows a conventional horizontal deflecting coil 101 and a conventional vertical deflecting coil 102. The horizontal deflecting coil 102 has deflection section 15 where copper wires run in parallel to a tube axis of the CRT, coil-connection-wire section 16 at a rear end towards the electron gun, and coil-connection-wire section 44 at the end towards the screen surface. In coil-connection-wire sections 16 and 44, copper wires run in a direction perpendicular to the tube axis of the CRT. Since having a direction identical to the direction of the electron beam emitted, Magnetic lines produced by the coil-connection-wire sections 16 and 44 do not contribute to a deflection of the electron beam.
FIG. 10 shows a main deflecting coil and a conventional sub-deflecting coil. Sub-deflecting coil 1 provided at the rear end of the main deflecting coil towards the electron gun receives magnetic line 17 generated by the coil-connection-wire section 16 at the electron gun side of the horizontal deflecting coil. Thus, an induced voltage is generated at the sub-deflection yoke 1, thereby causing noise in the picture.
The induced voltage that is generally called “CY cross talk” is about 19V when a voltage of 1,200V is applied to the horizontal deflecting coil. The vertical deflecting coil 31 has a similar coil-connection-wire section 103. But, the voltage applied to the vertical deflecting coil does not exceed 100V, and thus does not generate magnetic lines from the vertical deflecting coil to induce a voltage generating the noise.
When the correction of the screen position is not necessary, conventional centering magnets 3 inversely superpose the magnetic lines 14-1 generated inside the ring body 3a-1 and the magnetic lines 14-2 generated inside the ring body 3a-2 for canceling the lines, as shown in FIG. 8, thereby creating no magnetic line inside the ring body. However, it is practically difficult to completely eliminate the magnetic lines inside the ring body by superposing the centering magnets for the following reason.
FIG. 11 shows directions of resin that flows during the injection molding of the centering magnet. FIG. 12 shows the magnetic lines in the conventional centering magnet. As shown in FIG. 11, plastic resin mixed with ferromagnetic powder flows from port 18 along a direction of arrow 19 and reaches the knob portion 3c at a side opposite to the port 18. Alnico resin powder has a grain size of about 90 μm and is heavier than the resin. The resin including the ferromagnetic material powder at a higher density has a smaller viscosity, thus flowing more easily. Accordingly, the ferromagnetic material powder of high density gathers at the knob portion 3c at the side opposite to the gate 18. Upon such centering magnet being polarized, the magnetic lines is strong at a portion near the knob portion 3c, as shown in FIG. 12, including a high density of the ferromagnetic material powder.
FIG. 13 and FIG. 14 show the magnetic lines created by the conventional centering magnet. Even if the knob portions 3b and 3c of the magnets having magnetic lines asymmetrical to each other as described above are placed one on another for canceling the magnetic lines, their internal magnetic lines are not completely canceled and remains a 4-pole magnetic field. Accordingly, as shown in FIG. 14, electron beam 20 passing through the interior of the centering magnet receives force 22 due to the magnetic line 21 and thus has a spot deform into an oval shape, thus causing deterioration of a focus on the screen.